Characterization and control of the fiber-matrix interface in ceramic ...

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Conversely, the individual Nicalon filaments were impossible to
test. The number of dependent variables increased greatly with
decreasing size of the filaments. The most obvious of these were warping
of the filaments during the coating process and the influence of coating
microstructure.
The majority of the Nicalon filaments warped during
coating.
The problem is a combination of the thermal expansion mismatch
between the fiber and mounting fixture and the unforgiving forces of
gravity. Tensile loading of bent coated fibers resulted in premature and
catastrophic failure of the specimens.
The morphology and structure of the coatings greatly influenced the
testing of the small-diameter filaments. The grain size of the coating,
and perturbations at the coating surface affected the coating fracture.
The average grain size of CVD Sic deposited on Nicalon fibers at the
given experimental conditions is approximately 1 pm. Nucleation is rapid
and uniform at the untreated fiber surface; however, nucleation of Sic on
the surface of the carbon intermediate layer is somehow hindered.
This results in a coating having poorly distributed, large Sic grains.
Linear expressions for shear stress were used in the analysis of a
coated fiber placed in tension. As with the indentation method, morerealistic
shear-lag theories have been applied to the fiber-sheath model
(162-164).
These analyses account for shear stress buildup near the ends
of the fibers and the nonlinear decreases in stress away from the
fracture (Figure 11.5).
The shear-lag theories, however, may not be
accurate at higher stress levels (72).
Hany of the computations rely on
elastic behavior in the components, and as discussed, ceramic materials
exhibit very limited elastic response. Factors such as Poisson's